Many types of double boilers, steamers, pressure cookers and the like have been developed for warming a complete cooking of a variety of foods. In the low pressure types, such as double boilers or pans with non-sealed lids, steam accumulates under the lid and saturates the food. In addition, excess steam condenses on the underside of the lid and drips onto the food. As a result the food becomes soggy and may even tend to disintegrate. For some types of foods the excess humidity is undesirable, which limites the use of the equipment.
In warming or keeping warm such foods as cakes, rolls, bread, tortillas and the like, dry heat is sometimes used. Prolonged dry heat can dry out the food, but excess moisture is equally undesirable. Vegetables and fruits can tolerate more moisture, but an excess is still not desirable. A limited amount of humidity is effective in warming puddings and other hot deserts, to avoid formation of a crust which occurs in drying out. Meats and particularly fish require more steam, but still not an excess as produced in utensils now in use.
It would, therefore, be a great advantage to be able to warm food with a closely controlled degree of humidity, suited to the specific food, but without complex control or metering devices.